206 



THE SIXTH DAY. 



[chap. 



By the thirteenth day the cartilaginous skeleton is com- 

 pleted and the various muscles of the body can be made out 

 with tolerable clearness. 



Ossification begins according to Von Baer on the eighth 

 or ninth day by small deposits in the tibia, in the metacarpal 

 bones of the hind- limb, and in the scapula. On the eleventh or 

 twelfth day a multitude of points of ossification make their 

 appearance in the limbs, in the scapular and pelvic arches, in 

 the ribs, in the bodies of the cervical and dorsal vertebrae and 

 in the bones of the head, the centres of ossification of the 

 vertebral arches not being found till the thirteenth day. 



Fig. 62. 



, Diagram op the Venods Cieoulation at the Commencement of the 



I'lETH Day. 



II. heart. B. C. ductus Cuvieri. Into the ductus Cuvieri of each side fall /. the 

 jugular vein, Su. V. the superior vertebral, W. the vein from the wing and 

 C. the inferior cardinal vein. S. V. sinus venosus. Of. omphalo-mesaraic 

 vein. U. umbilical vein, which at this stage gives off branches to the 

 body-walls. V. (J. I. inferior vena cava. 



